A honeycomb structure is incorporated in an exhaust gas system or the like of a diesel engine as a trapping filter for exhaust gas as, for example, a diesel particulate filter (DPF) in order to trap and remove particulate matter contained in exhaust gas from a diesel engine or the like. Such a honeycomb structure has a problem in that, during use (when particulate matter is trapped and removed) and during regeneration (when particulate matter deposited inside a filter is burned and removed in order to reduce an increase in pressure loss caused by the particulate matter deposited inside the filter over time), a temperature rise in the whole honeycomb structure easily becomes nonuniform, and defects such as cracks are generated due to the thermal stress accompanying a local high-temperature rise. From the viewpoint of solving such a problem, there is suggested a honeycomb structure having reduced thermal stress by forming a bonded honeycomb segment body in which a plurality of honeycomb segments are integrally bonded at their bonding surfaces by means of a bonding material.
However, in such a honeycomb structure, the binding force between a honeycomb segment and the bonding material is not sufficient yet, which may become a bonding defect such as occurrence of peeling-off or cracking from the interface between them. In order to solve such a problem, there is suggested to provide an under layer or an intermediate layer between the honeycomb segment and the bonding material to improve the binding force and relax the stress at the interface (for example, refer to Patent Documents 1 and 2).
Further, if the honeycomb structure is used as a DPF to trap soot, and the DPF is regenerated, the temperature of the DPF may rise by burning of the trapped soot, and cracks may be generated due to the thermal stress caused at this time. Particularly if the porosity of the honeycomb structure is raised in order to lower the pressure loss of the DPF, thermal conductivity and heat capacity may be lowered, the temperature distribution in the honeycomb structure easily becomes nonuniform, and cracks may be generated due to the thermal stress caused at that time. Moreover, generally, in a case where the honeycomb structure is used for a catalyst carrier, a DPF, etc., a catalyst is carried in a partition wall using catalyst slurry. In this case, there is a problem in that the catalyst slurry permeates toward an outer surface from the inner surface of an outer wall, and only catalytic liquid of the slurry permeates into the bonding material, whereby the catalyst is concentrated at the interface between the outer wall and the bonding material.    [Patent Document 1] JP-A-2003-155908    [Patent Document 2] WO 2003/048072 pamphlet